1. Field of the Invention
The present invention relates to a new and useful improvement in container ends and more particularly to a thermoplastic end for a container having paperboard sidewalls. The invention further relates to a method of and apparatus for fabricating container components by inertial spinwelding.
2. Background of the Invention
Prior art paperboard sidewall containers, such as those used to package oil, orange juice, liquids and the like, or to package powders or other granular materials have typically utilized metal container ends attached to the paperboard sidewall by crimping or similar mechanical bonding. Such a container is expensive and complicated to construct, both in terms of the materials used and the fabrication methods utilized.
Plastic ends for such containers would be preferable but prior to this invention, no satisfactory method had been devised for attaching a plastic container end to a paperboard sidewall. Among the problems encountered are the relatively wide variations from nominal dimensions the paperboard sidewalls are produced in as well as the tendency for the plastic container ends to be dislodged from the container when a large force is applied to it in a direction normal to the plane of the container end, for example by dropping or shaking a full container. In addition, some plastic container ends exhibited a tendency to peel off or shear from the inside of the can wall.
As more fully discussed in copending Ser. No. 234,344 referenced above, Reissue U.S. Pat. No. 29,448 discloses methods and apparatus for intertial spinwelding of thermoplastic container parts. As disclosed in that patent, two axially mating thermoplastic container sections are respectively mounted upon axially aligned mandrels. One of the mandrels is temporarily coupled to a rotary drive means to bring that mandrel and the container part carried by the mandrel up to a predetermined rotative speed, at which time the rotary drive is disengaged, the rotary inertia developed maintaining the mandrel in rotation after the drive is disengaged. The two mandrels are then moved toward each other and the two container parts carried by the respective mandrels seat with each other. The friction developed by the relatively rotating container section heats the plastic material as it simultaneously brakes the relative rotation to melt the material to fuse the sections to each other when the relative rotation ceases and the sections are permitted to cool.
In Reissue U.S. Pat. No. 29,448, the two container sections being welded were both of a thermoplastic material. This fact is worthy of note in that in order to generate the frictional heat required to melt the plastic material, the mating sections of the container should preferably fit with each other with an interference fit. Where both sections are formed from the same thermoplastic material, the achievement of an interference fit of this type is not especially difficult in that the container section dimension is quite accurately established in the forming machine and any subsequent dimensional changes due to thermal expansion or contraction where the sections are stored for any substantial period of time prior to assembly normally affects both of the sections to substantially the same degree.
As alluded to above, in recent years, there has been substantial usage of containers in which the container body or side wall is formed primarily of paperboard or cardboard, usually sealed at the opposite ends by metal tops and bottoms. Cans for motor oil and frozen orange juice are typical examples of containers of this type. Where a paperboard container body is employed, it is often necessary to coat or line the interior of the paperboard body with some liquid tight material. Thermoplastic materials are frequently used for this purpose. Other containers, such as those used to hold powdered or solid materials, can be made using an uncoated paperboard body or sidewall.
Where the paperboard container body is lined with thermoplastic material, it has been proposed to employ a thermoplastic material for the container bottom which has led to the discovery that such bottoms could be spinwelded to the container body inasmuch as the container body has a layer of thermoplastic material on its interior surface. However, difficulties have been encountered in forming and maintaining the thermoplastic coated paperboard bodies within dimensional tolerances acceptable for such a spinwelding operation. In order to apply the thermoplastic liner to the paperboard, the thermoplastic is normally heated in order to bond it to the paperboard and subsequent cooling tends to shrink the material so that the container becomes undersized. Non-uniform shrinkage in storage also tends to occur, and the paperboard containers may, during preassembly handling and conveying operations, become slightly out of round.
Even in those instances where the paperboard sidewall is not coated with a thermoplastic fiber, many of the problems discussed above with regard to spinwelding are present. The primary concern, in either situation, is to insure that a bond of suitable integrity has been formed between the thermoplastic container end and the paperboard sidewall which will survive any expected forces which may be imposed on it.
A problem with all paperboard containers relates to the swelling of the paperboard with moisture content and the resultant problem of liquid "wicking up" through the paperboard with the attendant problem of leakage as well as the problem of loss of strength of the crimped metal-end container at the point of crimping where the liquid absorption is likely to occur.
Spinwelding plastic ends onto coated paperboard prevents wicking and leakage. On both coated and non-coated containers, a spinwelded container end is stronger than a crimped metal container end by a factor of two-to-one or higher.